New NIST Spectrometer Measures Single Photons with Great Precision

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A laser strikes a gas of atoms, which vibrate and block much of the light. A second laser is tuned to nearly the same wavelength as the first. The interference between the beams creates a narrow "hole" through which photons of a specific wavelength are transparent and can pass. Making fine adjustments to the second laser’s wavelength moves the hole, enabling highly precise measurements of the photon’s wavelength.

Credit: Irvine/NIST

Future communications networks that are less vulnerable to hacking could be closer to reality with an invention that measures the properties of single-photon sources with high accuracy.

Built by scientists at the National Institute of Standards and Technology (NIST), the device could help bring about “quantum communications” networks, which would use individual particles of light to send bits of information. Because each bit of information can be embedded in the quantum properties of a single photon, the laws of quantum mechanics make it difficult, if not impossible, for an enemy to intercept the message undetected.

Both the telecommunications and computer industries would like such networks to keep information secure. The NIST method may help overcome one of the technical barriers standing in their way by measuring photons’ spectral properties—essentially their color—10,000 times better than conventional spectrometers.

Individual photons have a limitation: They cannot travel through fiber-optic cables for more than about 100 kilometers (about 60 miles) without likely being absorbed. A quantum network able to handle worldwide communications would need periodic way stations that could catch photons and retransmit their information without loss. The NIST team’s invention could help such a “quantum repeater” interact effectively with photons.

Key to the operation of the quantum repeater would be a memory component that uses an ensemble of atoms to store the photon’s information briefly and retransmit it at the right moment. Its operation would involve an atom’s energy structure: As an atom catches the photon, the atom’s energy level rises to a higher state. At the desired moment, the atom returns to its original state and emits the energy as another photon.

Not just any photon can readily interact with this atom, though. It needs to be exactly the right color, or wavelength, to be absorbed by the atom. To make usable repeaters, engineers need to measure photons’ wavelengths far more precisely than conventional spectrometers can.

The NIST team goes past convention with a technique called electromagnetically induced transparency (EIT), which starts out by using atoms’ ability to block light of a specific wavelength.

Astronomers can tell what gases form the atmosphere of a far-off world because light passing through it makes the gas molecules vibrate at frequencies that block out light of particular colors, creating telltale dark lines in the light’s spectrum. EIT essentially creates a single dark line by beaming a laser at atoms whose vibrations block much of its light. A second laser, tuned to nearly the same wavelength as the first, is directed at the same atom and the interference between these two nearly identical beams alters the darkness. Instead of a simple dark line, it creates a line with a narrow transparent hole through which photons only of an extremely specific wavelength can pass.

By making fine adjustments to the second laser’s wavelength, the team found it could move the hole back and forth across the dark line’s width, giving them a way to make highly precise measurements of a passing photon’s wavelength.

To give a sense of how precise their spectrometer is, the team gave the example of a common laser pointer that shines in a single narrow color range, creating a pure-colored point on a screen. The typical spectrum width of a laser pointer is right around 1 terahertz (THz). The NIST invention can measure the color of a single-photon-level signal that has a spectrum 10 million times narrower than the laser pointer, resulting in a performance 10,000 times better than typical conventional spectrometers.

“Additionally, we can extend our EIT spectrometer’s performance to any other wavelength range using other processes developed by our group without sacrificing its spectral resolution, high wavelength accuracy and high detection sensitivity,” said Lijun Ma, an optical engineer on the NIST team. “We think this will give the industry the tool it needs to build effective quantum repeaters.”